For technological reasons, in the context of wideband (of the order of about ten gigahertz width for example) monitoring of electromagnetic signals, it is not generally possible to carry out sampling at a frequency meeting the Nyquist criterion, nor to process the data obtained by this sampling. This means that it is necessary to carry out sampling at frequencies lower than the Nyquist frequency, this creating problems with spectral overlap or aliasing.
If a single useful signal is present in the entirety of the analyzed band, this sampling technique does not create problems, neither as regards detection, since the signal is isolated in the aliased band, nor as regards signal analysis.
In contrast, if a plurality of signals are present simultaneously in the entire band, they may superpose in the aliased band, even if they are not in fact at the same frequency.
For very wideband systems for monitoring electromagnetic signals, such as for example communications signals or radar signals, it is not possible at the present time to simultaneously have a maximum probability of intercept and satisfactory analysis capabilities. The systems that perform these intercepts may be divided into two categories:
A first category corresponds to very wideband receivers. These receivers continuously cover the entire band to be analyzed and have a very high probability of intercept (POI) for strong signals, but are characterized by a low sensitivity and a very low ability to discriminate or analyze electromagnetic signals.
A second group corresponds to what are called “superheterodyne” narrowband receivers. These receivers, after multibit sampling of the band via a conventional method, allow refined analyses to be carried out on the signal (with a high sensitivity even allowing the modulation to be sought after discrete Fourier transform), but obviously suffer from a degraded POI since out of band signals are not processed. Sequencing functions that consist in determining the time to spend monitoring partial bands do exist but they only partially remedy this drawback.
In this context, it would be advantageous to provide a solution allowing the advantages of these two families of receiver to be combined. Such a solution would make it possible not only to permanently monitor a wideband but also to implement suitable two-or-more bit sampling in order to effectively process the received signals.
The conventional detectors found in the literature only take into account thermal noise. Thus, parasitic signals that are in fact themselves signals are not taken into account in the detection method.
Multibit sub-Nyquist wideband digital receivers are also known in the prior art, especially from U.S. Pat. No. 7,482,967. However, such systems do not treat possible problems with aliasing of other signals.